It sounds like you don't have a current mode balun, or maybe your balun
isn't working properly. If it's a voltage mode balun, the feedline will
act as part of the antenna and change the resonant frequency.
The 281 ft length you have, at the heights you described, should
resonant at 1.63 MHz with an impedance of 73 ohms. The 271 ft you
calculated should resonate at 1.68 MHz with an impedance of 75 ohms.
Sounds like my EZNEC simulations don't agree with your calculations. It
also seems that the use of a 4:1 balun is also pretty far off. 1:1 would
have been closer. If the antenna was higher, say add another 120 ft, it
would be a closer match for a 4:1 balun. You can also move the feed
point to get a different impedance. The dropping end causes some
changes. Compare what you built (181 ft = 1.63 MHz, 73 ohms) with the
same antenna as a flat top at 70 ft, resonant frequency = 1.65 MHz, 64 ohms.
Jerry, K4SAV
W4ZW wrote:
>
>I decided to try a OCF (Off Centerfed Dipole) dipole for the 160M contest
>this weekend rather than trying to put up the extended double bazooka I
>normally use. The bazooka worked well but was a single band antenna and I
>would replace it after any 160M contest with either a Carolina Windom or a
>80M delta loop. Anyway, after calculating the dimensions, I put up this OCF
>dipole using insulated #14 wire I got from the Wireman at the Orlando
>Hamcation. The center is at about 70' fed with coax and a 4:1 balun, with
>the short leg of 86' running to a 50' push-up Rohn mast that I secured in
>big Black Olive tree to the East. The other end runs West to the top of my
>50' flagpole for about 110' and then I sloped it down for the remaining 85'
>and tied it to the wooden walkway to the beach.
>
>Now my question is this. After making this thing longer than I thought I
>would need (271') so I could cut the ends to match, I discovered that it was
>resonant above the band! Actually about 2.1 MHz. Even with the insulated
>wire! I then added 25' that I considered to be an excessive additional
>amount to the long end, only to find that it only dropped it to about 1.950
>MHz.
>
>With insulated wire, and one end pretty close to ground (6') I though it
>would resonate fairly low so I could start cutting. I have another 50' Rohn
>push-up mast that I had already installed on the beach walkway but with
>these dimensions, it will be way short of where I need it to be located. I
>have my catamaran tied down on the beach another 70' away, and may try
>tying one end of the OCF to the top of the mast which is about 35' high.
>
>It seemed to tune OK using the built-in tuner of my FT-1000MP, and I could
>work the loud stations but a lot of the weaker guys just CQed in my face.
>(But of course, LP on 160Min a contest is a stretch.)
>
>Any suggestions or thoughts as to why this OCF is so long? Could it be too
>long such that I'm well into the next phase?
>
>Today is a beautiful day here in SW Florida and I should find the time to
>play with this later today.
>
>
>
>Jon Hamlet, W4ZW
>
>Casey Key Island, Florida
>"A little piece of paradise in the Gulf of Mexico"
>
>
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>
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